Abstract

Selectors are patterns that match
against elements in a tree, and as such form one of several technologies
that can be used to select nodes in an XML document. Selectors have been
optimized for use with HTML and XML, and are designed to be usable in
performance-critical code. They are a core component of CSS (Cascading Style Sheets), which
uses Selectors to bind style properties to elements in the document.

Selectors Level 4 describes the selectors that already exist in [SELECT], and further
introduces new selectors for CSS and other languages that may need them.

Status of this Document

This section describes the status of this document at the time of
its publication. Other documents may supersede this document. A list of
current W3C publications and the latest revision of this technical report
can be found in the W3C technical reports
index at http://www.w3.org/TR/.

Publication as a Working Draft does not imply endorsement by the W3C
Membership. This is a draft document and may be updated, replaced or
obsoleted by other documents at any time. It is inappropriate to cite this
document as other than work in progress.

The (archived) public
mailing list www-style@w3.org (see
instructions) is preferred
for discussion of this specification. When sending e-mail, please put the
text “selectors4” in the subject, preferably like this:
“[selectors4] …summary of comment…”

This module is a somewhat-unstable Working Draft. If you are
looking for a stable Selectors specification, use Selectors 3. Read
the CSS Snapshot for an overview
of the CSS development process. See the Selectors
Overview for a summary of additions to level 3.

The following features are at-risk and may be dropped during the CR
period if there is not sufficient implementer interest: the reference
combinator, the column combinator, the ‘:invalid-drop’ and ‘:valid-drop’ pseudo-classes.

1. Introduction

A selector is a boolean predicate that takes an element in a tree
structure and tests whether the element matches the selector or not.

These expressions may be used for many things:

directly on an element to test whether it matches some criteria, such
as in the Element.matches() function defined in [SELECTORS-API2]

applied to an entire tree of elements to filter it into a set of
elements that match the criteria, such as in the
document.findAll() function defined in [SELECTORS-API2] or the
selector of a CSS style rule.

used "in reverse" to generate markup that would match a given
selector, such as in HAML or Zen Coding

1.1. Module Interactions

This module replaces the definitions of and extends the set of selectors
defined for CSS in [SELECT] and [CSS21].

Pseudo-element selectors, which define abstract elements in a rendering
tree, are not part of this specification: their generic syntax is
described here, but, due to their close integration with the rendering
model and irrelevance to other uses such as DOM queries, they will be
defined in other modules.

2. Selectors Overview

This section is non-normative, as it merely summarizes the following
sections.

A Selector represents a structure. This structure can be used as a
condition (e.g. in a CSS rule) that determines which elements a selector
matches in the document tree, or as a flat description of the HTML or XML
fragment corresponding to that structure.

Selectors are used in many different contexts, with wildly varying
performance characteristics. Some powerful selectors are unfortunately too
slow to realistically include in the more performance-sensitive contexts.
To accommodate this, two profiles of the Selectors spec are defined:

fast

The fast profile is appropriate
for use in any context, including dynamic browser CSS selector matching.
It includes every selector defined in this document, except for:

The complete profile is
appropriate for contexts which aren't extremely performance sensitive.
For example, implementations of the Selectors API specification [SELECTORS-API] should use
the ‘complete’
profile. It includes all of the selectors defined in this document.

CSS implementations conformant to Selectors Level 4 must use the ‘fast’ profile for CSS
selection.

A combinator is punctuation that represents a
particular kind of relationship between the compound selectors on either side. Combinators
in Selectors level 4 include: whitespace, "greater-than sign"
(U+003E, >), "plus sign" (U+002B, +)
and "tilde" (U+007E, ~). White space may appear between a combinator and the
simple selectors around it.

3.2. Determining the Subject of a
Selector

The elements of a document tree that are represented by a selector are
the subjects of the selector.

By default, the subjects of a selector are the elements represented by
the last compound selector in the selector.
Thus a selector consisting of a single compound
selector represents any element satisfying its requirements
Prepending another compound selector and a
combinator to a sequence imposes additional matching constraints, so the
subjects of the selector are always a subset of the elements represented
by the last compound selector.

As a feature of the complete
Selectors profile, the subject of the selector can be explicitly
identified by prepending an exclamation mark (!) to one of the compound selectors in a selector. Although the
element structure that the selector represents is the same with or without
the exclamation mark, indicating the subject in this way can change which
compound selector represents the subject in
that structure.

Should the exclamation mark be prepended or appended to the
subject? Or both? Or prepend two, to avoid the "! = not" issue?

For example, the following selector represents a list item
LI unique child of an ordered list OL:

OL > LI:only-child

However the following one represents an ordered list OL
having a unique child, that child being a LI:

!OL > LI:only-child

The tree structures represented by these two selectors are the same,
but the subjects of the selectors are not.

3.3. Scoped Selectors

Some host applications may choose to scope selectors
to a particular subtree of the document. The root of the scoping subtree
is called the scoping element, and is
in-scope. When scoped selectors are used, it forms the contextual reference element
set and matches the :scope pseudo-class.

There are three methods of scoping selectors:

scope-contained
selectors

With this method of scoping, selectors match as if the scoping element were the root of the
document: all compound selectors must
match elements within the scope. (The :root pseudo-class, however, still
only matches the actual root of the document.)

scope-filtered
selectors

With this method of scoping, a selector matches if the subject
of the selector is within the scope, even if other components of the
selector are outside the scope.

scope-relative
selectors

With this method of scoping, ":scope " (the :scope pseudo-class followed by a
space) is implied at the beginning of each complex
selector that does not already contain the :scope pseudo-class. This allows
the selector to begin syntactically with a combinator. The scoping element matches
this implied :scope selector,
but does not limit which elements match.

Scope-relative selectors must be absolutized before using them for matching.

It might be necessary (for, e.g.
::distributed() or documentFragment.find()),
to split the concept of scope-relative selector into multiple concepts.

For example, the element.querySelector() function defined
in [SELECTORS-API2] allows the
author to define a scope-filtered
selector, while the similar element.find function defined in
the same spec uses scope-relative
selectors.

Otherwise, if the selector does not contain any instance of the :scope pseudo-class (either at the
top-level or as an argument to a functional pseudo-class), prepend :scope followed by the descendant combinator.

Otherwise, the selector is already absolute.

To absolutize a scope-relative selector
list, absolutize each scope-relative selector in the list.

3.4. Pseudo-classes

The pseudo-class concept is introduced to
permit selection based on information that lies outside of the document
tree or that can be awkward or impossible to express using the other
simple selectors.

A pseudo-class always consists of a
“colon” (:) followed by the name of the pseudo-class and, for functional pseudo-classes, by one or
more arguments between parentheses (similar to CSS functions). White space
is optionally allowed between the parentheses and the argument, but not
between the pseudo-class name and the parentheses. If arguments are
separated by commas, white space is optionally allowed before/after each
comma.

Dynamic pseudo-classes classify
elements on characteristics other than their name, attributes, or content,
but rather on characteristics that cannot be deduced from the document
tree. They do not appear in or modify the document source or document
tree.

3.5. Pseudo-elements

Pseudo-elements create
abstractions about the document tree beyond those specified by the
document language. For instance, document languages do not offer
mechanisms to access the first letter or first line of an element's
content. Pseudo-elements allow
authors to refer to this otherwise inaccessible information. Pseudo-elements may also provide authors
a way to refer to content that does not exist in the source document
(e.g., the ::before and ::afterpseudo-elements give access to generated
content in CSS [CSS21]).

This :: notation was chosen in order to establish a
discrimination between pseudo-classes
(which subclass existing elements) and pseudo-elements (which are elements not
represented in the document tree). However, for compatibility with
existing style sheets, user agents must also accept the previous one-colon
notation for pseudo-elements
introduced in CSS levels 1 and 2 (namely, :first-line,
:first-letter, :before and :after).
This compatibility notation is not allowed any other pseudo-elements.

A future version of this specification may allow multiple
pseudo-elements per selector.

For example, the :hover
pseudo-class specifies that it can apply to any pseudo-element, i.e.
::first-line:hover will match when the first line is
hovered. However, since neither :focus nor
::first-line define that :focus can apply to
::first-line, the selector ::first-line:focus
will never match anything.

The host language defines which pseudo-elements exist and their meaning.
For CSS, [CSS21]
defines the ::before, ::after, ::first-line and ::first-letter
pseudo-elements.

3.6. Characters and case
sensitivity

All Selectors syntax is case-insensitive within the ASCII range (i.e.
[a-z] and [A-Z] are equivalent), except for the following parts, which are
not under the control of Selectors: the case-sensitivity of document
language element names, attribute names, and attribute values depends on
the document language. For example, in HTML,
element names are case-insensitive, but in XML, they are
case-sensitive. Case sensitivity of namespace prefixes is defined in [CSS3NAMESPACE]. Case
sensitivity of language ranges is
defined in the :lang() section.

White space in Selectors consists of the
characters SPACE (U+0020), TAB (U+0009), LINE FEED (U+000A), CARRIAGE
RETURN (U+000D), and FORM FEED (U+000C) can occur in whitespace. Other
space-like characters, such as EM SPACE (U+2003) and IDEOGRAPHIC SPACE
(U+3000), are never part of white space.

Characters in Selectors can be escaped with a backslash according to the
same escaping
rules as CSS. [CSS21] Note that escaping a
character "cancels out" any special meaning it may have in Selectors. For
example, the selector ‘#foo>a’ contains a
combinator, but ‘#foo\>a’ instead selects an
element with the id foo>a.

3.7. Namespaces

Certain selectors support namespace prefixes. The mechanism by which
namespace prefixes are declared should be specified
by the language that uses Selectors. If the language does not specify a
namespace prefix declaration mechanism, then no prefixes are declared. In
CSS, namespace prefixes are declared with the @namespace
rule. [CSS3NAMESPACE]

3.8. Invalid Selectors and Error
Handling

User agents must observe the rules for handling invalid
selectors:

a parsing error in a selector, e.g. an unrecognized token or a token
which is not allowed at the current parsing point, causes that selector
to be invalid

4. Logical
Combinations

4.1. Selector Lists

A comma-separated list of selectors represents the union of all elements
selected by each of the individual selectors in the selector list. (A comma is U+002C.) For example, in
CSS when several selectors share the same declarations, they may be
grouped into a comma-separated list. White space may appear before and/or
after the comma.

CSS example:

In this example, we condense three rules with identical declarations
into one. Thus,

Warning: the equivalence is true in this example
because all the selectors are valid selectors. If just one of these
selectors were invalid, the entire selector
list would be invalid. This would invalidate the rule for all
three heading elements, whereas in the former case only one of the three
individual heading rules would be invalidated.

The :matches() pseudo-class
may not be nested within itself or within :not():
:matches(:matches(...)) and :not(:matches(...))
are invalid. Additionally, pseudo-elements cannot be represented by the
matches-any pseudo-class; they are not valid within :matches().

Default namespace declarations do not affect any “implied” universal
selectors within a :matches()
pseudo-class.

For example, following selector matches any element that is being
hovered or focused, regardless of its namespace. In particular, it is not
limited to only matching elements in the default namespace that are being
hovered or focused.

*|*:matches(:hover, :focus)

The following selector, however, represents only hovered or focused
elements that are in the default namespace, because it uses an explicit
universal selector within the :matches() notation:

A negation may not be nested within itself or within :matches():
:not(:not(...)) and :matches(:not(...)) are
invalid. Additionally, pseudo-elements cannot be represented by the
negation pseudo-class; they are not valid within :not().

For example, the following selector matches all button
elements in an HTML document that are not disabled.

button:not([DISABLED])

The following selector represents all but FOO elements.

*:not(FOO)

The following compound selector represents all HTML elements except
links.

html|*:not(:link):not(:visited)

Default namespace declarations do not affect the subject of any selector
within a negation pseudo-class unless the argument is an explicit
universal selector or a type selector. (See :matches() for examples.)

Note: the :not() pseudo allows useless
selectors to be written. For instance :not(*|*), which represents no
element at all, or foo:not(bar), which is equivalent to
foo but with a higher specificity.

5. Elemental selectors

5.1. Type (tag name)
selector

A type selector is the name of a document
language element type written using the syntax of CSS qualified
names[CSS3NAMESPACE]. A type
selector represents an instance of the element type in the document tree.

Example:

The following selector represents an h1 element in the
document tree:

h1

5.1.1. Type selectors and
namespaces

Type selectors allow an optional namespace component: a namespace prefix
that has been previously declared may be prepended
to the element name separated by the namespace separator "vertical
bar" (U+007C, |). (See, e.g., [XML-NAMES] for the use of
namespaces in XML.)

The namespace component may be left empty (no prefix before the
namespace separator) to indicate that the selector is only to represent
elements with no namespace.

An asterisk may be used for the namespace prefix, indicating that the
selector represents elements in any namespace (including elements with no
namespace).

Element type selectors that have no namespace component (no namespace
separator) represent elements without regard to the element's namespace
(equivalent to "*|") unless a default namespace has been declared for namespaced selectors (e.g. in CSS, in the
style sheet). If a default namespace has been declared, such selectors
will represent only elements in the default namespace.

A type selector containing a namespace prefix that has not been
previously declared for namespaced selectors is an
invalid selector.

In a namespace-aware client, the name part of element type selectors
(the part after the namespace separator, if it is present) will only match
against the local part of
the element's qualified
name.

In summary:

ns|E

elements with name E in namespace ns

*|E

elements with name E in any namespace, including those without a
namespace

|E

elements with name E without a namespace

E

if no default namespace has been declared for
selectors, this is equivalent to *|E. Otherwise it is equivalent to ns|E
where ns is the default namespace.

The first rule (not counting the @namespace at-rule) will
match only h1 elements in the "http://www.example.com"
namespace.

The second rule will match all elements in the "http://www.example.com"
namespace.

The third rule will match only h1 elements with no
namespace.

The fourth rule will match h1 elements in any namespace
(including those without any namespace).

The last rule is equivalent to the fourth rule because no default
namespace has been defined.

5.2. Universal selector

The universal selector, written as a
CSS qualified
name[CSS3NAMESPACE] with an
asterisk (* U+002A) as the local name, represents the
qualified name of any element type. It represents any single element in
the document tree in any namespace (including those without a namespace)
if no default namespace has been specified for selectors. If a default
namespace has been specified, see Universal selector
and Namespaces below.

If a universal selector represented by * (i.e. without a
namespace prefix) is not the only component of a compound selector or is immediately followed
by a pseudo-element, then the
* may be omitted and the universal selector's presence
implied.

Examples:

*[hreflang|=en] and [hreflang|=en] are
equivalent,

*.warning and .warning are equivalent,

*#myid and #myid are equivalent.

Note: it is recommended that the
* not be omitted, because it decreases the potential
confusion between, for example, div
:first-child and div:first-child. Here, div *:first-child is more readable.

5.2.1. Universal selector and
namespaces

The universal selector allows an optional namespace component. It is
used as follows:

ns|*

all elements in namespace ns

*|*

all elements

|*

all elements without a namespace

*

if no default namespace has been specified, this is equivalent to *|*.
Otherwise it is equivalent to ns|* where ns is the default namespace.

A universal selector containing a namespace prefix that has not been
previously declared is an invalid selector.

6. Attribute selectors

Selectors allow the representation of an element's attributes. When a
selector is used as an expression to match against an element, an attribute selector must be considered to match
an element if that element has an attribute that matches the attribute
represented by the attribute selector.

6.1. Attribute
presence and value selectors

Represents an element with the att attribute, whatever
the value of the attribute.

[att=val]

Represents an element with the att attribute whose value
is exactly "val".

[att~=val]

Represents an element with the att attribute whose value
is a whitespace-separated list of words, one of
which is exactly "val". If "val" contains whitespace, it will never
represent anything (since the words are separated by spaces).
Also if "val" is the empty string, it will never represent anything.

[att|=val]

Represents an element with the att attribute, its value
either being exactly "val" or beginning with "val" immediately followed
by "-" (U+002D). This is primarily intended to allow language subcode
matches (e.g., the hreflang attribute on the a element in HTML) as described in BCP 47 ([BCP47]) or its
successor. For lang (or xml:lang) language
subcode matching, please see the
:lang pseudo-class.

The following attribute selector represents an h1 element
that carries the title attribute, whatever its value:

h1[title]

In the following example, the selector represents a span
element whose class attribute has exactly the value
"example":

span[class="example"]

Multiple attribute selectors can be used to represent several
attributes of an element, or several conditions on the same attribute.
Here, the selector represents a span element whose
hello attribute has exactly the value "Cleveland" and whose
goodbye attribute has exactly the value "Columbus":

span[hello="Cleveland"][goodbye="Columbus"]

The following CSS rules illustrate the differences between "=" and
"~=". The first selector would match, for example, an a element with the value "copyright copyleft
copyeditor" on a rel attribute. The second selector would
only match an a element with an
href attribute having the exact value "http://www.w3.org/".

a[rel~="copyright"] { ... }
a[href="http://www.w3.org/"] { ... }

The following selector represents an a
element whose hreflang attribute is exactly "fr".

a[hreflang=fr]

The following selector represents an a
element for which the value of the hreflang attribute begins
with "en", including "en", "en-US", and "en-scouse":

a[hreflang|="en"]

The following selectors represent a DIALOGUE element
whenever it has one of two different values for an attribute
character:

DIALOGUE[character=romeo]
DIALOGUE[character=juliet]

6.2. Substring
matching attribute selectors

Three additional attribute selectors are provided for matching
substrings in the value of an attribute:

[att^=val]

Represents an element with the att attribute whose value
begins with the prefix "val". If "val" is the empty string then the
selector does not represent anything.

[att$=val]

Represents an element with the att attribute whose value
ends with the suffix "val". If "val" is the empty string then the
selector does not represent anything.

[att*=val]

Represents an element with the att attribute whose value
contains at least one instance of the substring "val". If "val" is the
empty string then the selector does not represent anything.

The following selector represents an HTML object,
referencing an image:

object[type^="image/"]

The following selector represents an HTML anchor a with an href attribute whose
value ends with ".html".

a[href$=".html"]

The following selector represents an HTML paragraph with a
title attribute whose value contains the substring "hello"

p[title*="hello"]

6.3. Case-sensitivity

By default case-sensitivity of attribute names and values in selectors
depends on the document language. To match attribute values
case-insensitively regardless of document language rules, the attribute
selector may include the identifier i before the closing
bracket (]). When this flag is present, UAs must match the
attribute's value case-insensitively within the ASCII range.

The following rule will style the frame attribute when it
has a value of hsides, whether that value is represented as
hsides, HSIDES, hSides, etc. even
in an XML environment where attribute values are case-sensitive.

[frame=hsides i] { border-style: solid none; }

6.4. Attribute selectors and
namespaces

The attribute name in an attribute selector is given as a CSS qualified
name: a namespace prefix that has been previously declared may be prepended to the attribute name
separated by the namespace separator "vertical bar"
(|). In keeping with the Namespaces in the XML
recommendation, default namespaces do not apply to attributes, therefore
attribute selectors without a namespace component apply only to attributes
that have no namespace (equivalent to "|attr"). An asterisk
may be used for the namespace prefix indicating that the selector is to
match all attribute names without regard to the attribute's namespace.

An attribute selector with an attribute name containing a namespace
prefix that has not been previously declared is an
invalid selector.

The first rule will match only elements with the attribute
att in the "http://www.example.com" namespace with the value
"val".

The second rule will match only elements with the attribute
att regardless of the namespace of the attribute (including
no namespace).

The last two rules are equivalent and will match only elements with the
attribute att where the attribute is not in a namespace.

6.5. Default attribute values
in DTDs

Attribute selectors represent attribute values in the document tree. How
that document tree is constructed is outside the scope of Selectors. In
some document formats default attribute values can be defined in a DTD or
elsewhere, but these can only be selected by attribute selectors if they
appear in the document tree. Selectors should be designed so that they
work whether or not the default values are included in the document tree.

For example, a XML UA may, but is not required to, read an
“external subset” of the DTD, but is required to look for
default attribute values in the document's “internal subset”. (See,
e.g., [XML10] for
definitions of these subsets.) Depending on the UA, a default attribute
value defined in the external subset of the DTD might or might not appear
in the document tree.

A UA that recognizes an XML namespace may, but is not required to use
its knowledge of that namespace to treat default attribute values as if
they were present in the document. (For example, an XHTML UA is not
required to use its built-in knowledge of the XHTML DTD. See, e.g., [XML-NAMES] for
details on namespaces in XML 1.0.)

Note: Typically, implementations choose to
ignore external subsets. This corresponds to the behaviour of
non-validating processors as defined by the XML specification.

Example:

Consider an element EXAMPLE with an attribute
radix that has a default value of "decimal".
The DTD fragment might be

Here, because the selector EXAMPLE[radix=octal] is more
specific than the type selector alone, the style declarations in the
second rule will override those in the first for elements that have a
radix attribute value of "octal". Care has to
be taken that all property declarations that are to apply only to the
default case are overridden in the non-default cases' style rules.

6.6. Class selectors

The class selector is given as a full stop
(. U+002E) immediately followed by an identifier. It represents an element
belonging to the class identified by the identifier, as defined by the
document language. For example, in [HTML5], [SVG11], and [MATHML] membership in a class is
given by the class attribute: in these languages it is
equivalent to the ~= notation applied to the local
class attribute (i.e.
[class~=identifier]).

CSS examples:

We can assign style information to all elements with
class~="pastoral" as follows:

*.pastoral { color: green } /* all elements with class~=pastoral */

or just

.pastoral { color: green } /* all elements with class~=pastoral */

The following assigns style only to H1 elements with
class~="pastoral":

H1.pastoral { color: green } /* H1 elements with class~=pastoral */

Given these rules, the first H1 instance below would not
have green text, while the second would:

<H1>Not green</H1>
<H1 class="pastoral">Very green</H1>

The following rule matches any P element whose
class attribute has been assigned a list of whitespace-separated values that includes both
pastoral and marine:

p.pastoral.marine { color: green }

This rule matches when class="pastoral blue aqua marine"
but does not match for class="pastoral blue".

Note: Because CSS gives considerable power
to the "class" attribute, authors could conceivably design their own
"document language" based on elements with almost no associated
presentation (such as DIV and SPAN in HTML) and
assigning style information through the "class" attribute. Authors should
avoid this practice since the structural elements of a document language
often have recognized and accepted meanings and author-defined classes may
not.

Note: If an element has multiple class
attributes, their values must be concatenated with spaces between the
values before searching for the class. As of this time the working group
is not aware of any manner in which this situation can be reached,
however, so this behavior is explicitly non-normative in this
specification.

6.7. ID selectors

Document languages may contain attributes that are declared to be of
type ID. What makes attributes of type ID special is that no two such
attributes can have the same value in a conformant document, regardless of
the type of the elements that carry them; whatever the document language,
an ID typed attribute can be used to uniquely identify its element. In
HTML all ID attributes are named id; XML applications may
name ID attributes differently, but the same restriction applies. Which
attribute on an element is considered the “ID attribute“ is defined by
the document language.

An ID selector consists of a “number sign”
(U+0023, #) immediately followed by the ID value, which must
be a CSS identifier.
An ID selector represents an element instance that has an identifier that
matches the identifier in the ID selector. (It is possible in
non-conforming documents for multiple elements to match a single ID
selector.)

In Quirks Mode, we accept all hash tokens, not just ones
whose value matches the ident syntax. Should we change the Standards Mode
to do that, too? Note that HTML5 loosened the definition of valid ids to
allow things starting with numbers, etc.

Examples:

The following ID selector represents an h1 element whose
ID-typed attribute has the value "chapter1":

h1#chapter1

The following ID selector represents any element whose ID-typed
attribute has the value "chapter1":

#chapter1

The following selector represents any element whose ID-typed attribute
has the value "z98y".

*#z98y

Note: In XML 1.0 [XML10], the information about which
attribute contains an element's IDs is contained in a DTD or a schema.
When parsing XML, UAs do not always read the DTD, and thus may not know
what the ID of an element is (though a UA may have namespace-specific
knowledge that allows it to determine which attribute is the ID attribute
for that namespace). If a style sheet author knows or suspects that a UA
may not know what the ID of an element is, he should use normal attribute
selectors instead: [name=p371] instead of #p371.

If an element has multiple ID attributes, all of them must be treated as
IDs for that element for the purposes of the ID selector. Such a situation
could be reached using mixtures of xml:id, DOM3 Core, XML DTDs, and
namespace-specific knowledge.

7. Location Pseudo-classes

The :any-link pseudo-class represents an
element that acts as the source anchor of a hyperlink. For example, in [HTML5], any
<a>, <area>, or <link>
elements with an href attribute are hyperlinks, and thus match
:any-link. It matches an element if the element would
match :link or :visited, equivalent to ‘:matches(:link, :visited)’.

The :local-link pseudo-class allows
authors to style hyperlinks based on
the users current location within a site and to differentiate
site-internal versus site-external links.

The (non-functional) :local-link pseudo-class
represents an element that is the source anchor of a hyperlink whose
target's absolute URL matches the element's own document URL. Any fragment
identifiers are stripped before matching the document's URL against the
link's URL; otherwise all portions of the URL are considered.

For example, the following rule prevents links targetting the current
page from being underlined when they are part of the navigation list:

nav :local-link { text-decoration: none; }

As a functional pseudo-class, :local-link() can also accept
a non-negative integer as its sole argument, which, if the document's URL
belongs to a hierarchical scheme, indicates the number of path levels to
match:

‘:local-link(0)’ represents a link element
whose target is in the same domain as the document's URL

‘:local-link(1)’ represents a link element
whose target has the same domain and first path segment

‘:local-link(2)’ represents a link element
whose target has the same domain, first, and second path segments

etc.

The following example styles all site-external links with a dashed
underline.

:not(:local-link(0)) { text-decoration-style: dashed; }

Path segments are portions of the URL's path that are separated by
forward slashes (/). If a segment is missing from the document's URL, a
pseudo-class requiring that segment to match does not match anything.
However, an empty final path segment is ignored.

So, given the links:

<a href="http://www.example.com">Home</a>

<a href="http://www.example.com/2011">2011</a>

<a href="http://www.example.com/2011/03">March</a>

<a
href="http://www.example.com/2011/03/">March</a>

<a href="http://www.example.com/2011/03/21">21
March</a>

<a
href="https://www.example.com/2011/03/">March</a>

<a href="http://example.com/2011/03">March</a>

and the styles:

a:local-link {...}

a:local-link(0) {...}

a:local-link(1) {...}

a:local-link(2) {...}

a:local-link(3) {...}

If the document's URL is http://www.example.com/2011/03/:

Link 1 would receive Style B

Link 2 would receive Styles B and C

Link 3 would receive Styles B, C, and D

Link 4 would also receive Styles A, B, C, and D

Link 5 would receive Styles B, C, and D

Link 6 would receive Styles B, C, and D

Link 7 would remain unstyled

Style E would not be applied to anything

The scheme, username, password, port, query string, and fragment
portions of the URL are not considered when matching against
:local-link(n). If the document's URL does not
belong to a hierarchical scheme, the functional pseudo-class matches
nothing.

Should a :local-link(2) match a link from the
document http://example.com/foo to itself? (This would make
Style 5 apply to Link 4.) (Relatedly, should a link from a document at an
opaque URL to itself also match?)

The :scope pseudo-class represents any
element that is in the contextual
reference element set. This is is a (potentially empty)
explicitly-specified set of elements, such as that specified by the
querySelector() call in [SELECTORS-API2], or the
parent element of a scoped
<style> element in [HTML5], which is used to "scope" a
selector so that it only matches within a subtree.

If no contextual reference element set is given, :scope is equivalent to :root. Specifications intending for
this pseudo-class to match specific elements rather than the document's
root element must define a contextual reference element
set.

8. User Action
Pseudo-classes

Interactive user agents sometimes change the rendering in response to
user actions. Selectors provides three pseudo-classes for the selection of
an element the user is acting on. (In non-interactive user agents, these
pseudo-classes are valid, but never match any element.)

These pseudo-classes are not mutually exclusive. An element may match
several pseudo-classes at the same time.

The :hover pseudo-class applies while the
user designates an element with a pointing device, but does not
necessarily activate it. For example, a visual user agent could apply this
pseudo-class when the cursor (mouse pointer) hovers over a box generated
by the element. Interactive user agents that cannot detect hovering due to
hardware limitations (e.g., a pen device that does not detect hovering)
are still conforming.

Host languages may define additional ways in which an element can match
:hover. For example, [HTML5] defines a
<label> element as matching
:hover when its labelled control is hovered.

Note: Since the ‘:hover’ state can apply
to an element because its child is designated by a pointing device, then
it is possible for ‘:hover’ to apply to an element that is not
underneath the pointing device.

The :active pseudo-class applies while an
element is being activated by the user. For example, between the times the
user presses the mouse button and releases it. On systems with more than
one mouse button, :active
applies only to the primary or primary activation button (typically the
"left" mouse button), and any aliases thereof.

8.4. The drag-and-drop
pseudo-classes

The drag-and-drop pseudo-classes apply while the user is ”dragging“
or otherwise conceptually carrying an item for which the element is a
valid drop target.

The :active-drop-target pseudo-class
represents an element that is the current drop target for an item that is
currently being dragged in a drag-and-drop interface.

The :valid-drop-target pseudo-class
represents an element that is a possible drop target for an item that is
currently being dragged in a drag-and-drop interface.

The :invalid-drop-target
pseudo-class represents an element that is a possible drop target, but
does not accept the item that is currently being dragged in a
drag-and-drop interface.

For example, [HTML5] defines the dropzone
attribute, which allows an author to declare an element as a "drop
target", and declare what kinds of data the element is willing to accept
from drag-and-drop.

The ‘:valid-drop-target’ pseudo-class could
be used, for example, to highlight all the valid drop targets for the
item being dragged.

:valid-drop-target { box-shadow: 0 0 5px yellow; }

Meanwhile the ‘:active-drop-target’
pseudo-class can be used to designate the drop-zone that will receive the
dragged item when dropped.

:active-drop-target { outline: solid red; }

The CSSWG would like to shorten/clarify the names of these
pseudo-classes. Comments are welcome. One possibility would be to simply
drop ‘-target’ from the current names.
Other suggestions being considered include:

Set A

Set B

Set C

:active-drop

:drop

:current-drop

:drop

:can-drop

:valid-drop

:no-drop

:no-drop

:invalid-drop

9. Time-dimensional
Pseudo-classes

These pseudo-classes classify elements with respect to the
currently-displayed or active position in some timeline, such as during
speech rendering of a document, or during the display of a video using
WebVTT to render subtitles.

The :current pseudo-class represents the
element, or an ancestor of the element, that is currently being displayed.

Its alternate form :current(), like :matches(), takes a list of compound selectors as its argument: it
represents the :current element
that matches the argument or, if that does not match, the innermost
ancestor of the :current
element that does. (If neither the :current element nor its ancestors
match the argument, then the selector does not represent anything.)

For example, the following rule will highlight whichever paragraph or
list item is being read aloud in a speech rendering of the document:

:current(p, li, dt, dd) {
background: yellow;
}

9.2. The past-element
pseudo-class :past

The :past pseudo-class represents any element that is
defined to occur entirely prior to a :current element. For example, the
WebVTT spec defines the :past pseudo-class relative
to the current playback position of a media element. If a time-based
order of elements is not defined by the document language, then this
represents any element that is a (possibly indirect) previous sibling of a
:current element.

10. Linguistic
Pseudo-classes

The :dir() pseudo-class allows the author to
write selectors that represent an element based on its directionality as
determined by the document language. For example, [HTML5] defines how to
determine the directionality of an element, based on a combination of
the dir attribute, the surrounding text, and other factors.
The :dir() pseudo-class does not
select based on stylistic states—for example, the CSS ‘direction’ property does not affect whether it
matches.

The pseudo-class :dir(ltr) represents an element that has a
directionality of left-to-right (ltr). The pseudo-class
:dir(rtl) represents an element that has a directionality of
right-to-left (rtl). The argument to :dir() must be a single identifier,
otherwise the selector is invalid. White space is optionally allowed
between the identifier and the parentheses. Values other than
ltr and rtl are not invalid, but do not match
anything. (If a future markup spec defines other directionalities, then
Selectors may be extended to allow corresponding values.)

The difference between :dir(C) and [dir=C] is
that [dir=C] only performs a comparison against a given
attribute on the element, while the :dir(C) pseudo-class uses
the UAs knowledge of the document's semantics to perform the comparison.
For example, in HTML, the directionality of an element inherits so that a
child without a dir attribute will have the same
directionality as its closest ancestor with a valid dir
attribute. As another example, in HTML,
an element that matches [dir=auto] will match
either :dir(ltr) or :dir(rtl) depending on the
resolved directionality of the elements as determined by its contents. [HTML5]

If the document language specifies how the (human) content language of
an element is determined, it is possible to write selectors that represent
an element based on its language. The :lang()
pseudo-class represents an element that is in one of the languages listed
in its argument. It accepts a comma-separated list of one or more language ranges as its argument. Each
language range in :lang() must be a valid CSS identifier[CSS21] or consist
of an asterisk (* U+002A) immediately followed by an identifier beginning
with an ASCII hyphen (U+002D) for the selector to be valid.

The language of an element is defined by the
document language. For example, in HTML [HTML401], the language is determined by a combination of the
lang attribute, information from meta elements,
and possibly also the protocol (e.g. from HTTP headers). XML languages can
use the xml:lang attribute to indicate language information
for an element.

The element's language matches a language range if the element's language (normalized to BCP 47 syntax if
necessary) matches the given language
range in an extended filtering operation per [RFC4647]Matching of Language Tags (section 3.3.2). The matching is
performed case-insensitively within the ASCII range. The language range does not need to be a
valid language code to perform this comparison.

The two following selectors represent an HTML document that is in
Belgian French or German. The two next selectors represent q
quotations in an arbitrary element in Belgian French or German.

html:lang(fr-be)
html:lang(de)
:lang(fr-be) > q
:lang(de) > q

One difference between :lang(C) and the ‘|=’ operator is that the ‘|=’ operator only performs a comparison against a given
attribute on the element, while the :lang(C) pseudo-class
uses the UAs knowledge of the document's semantics to perform the
comparison.

In this HTML example, only the BODY matches [lang|=fr]
(because it has a LANG attribute) but both the BODY and the P match
:lang(fr) (because both are in French). The P does not match
the [lang|=fr] because it does not have a LANG attribute.

<body lang=fr>
<p>Je suis français.</p>
</body>

Another difference between :lang(C) and the ‘|=’ operator is that :lang(C) performs
implicit wildcard matching. For example, :lang(de-DE) will
match all of ‘de-DE’, ‘de-DE-1996’, ‘de-Latn-DE’,
‘de-Latf-DE’, ‘de-Latn-DE-1996’, whereas of those
[lang|=de-DE] will only match ‘de-DE’ and ‘de-DE-1996’.

To perform wildcard matching on the first subtag (the primary
language), an asterisk must be used: *-CH will match all of
‘de-CH’, ‘it-CH’,
‘fr-CH’, and ‘rm-CH’.

Note that asterisks are not allowed anywhere else in :lang()‘s language
range syntax: they only have meaning, and are therefore only allowed, at
the beginning.

Wildcard language matching is new in Level 4.

11. The Input Pseudo-classes

The pseudo-classes in this section mostly apply to elements that take
user input, such as HTML’s <input> element.

Note: CSS properties that might affect a
user’s ability to interact with a given user interface element do not
affect whether it matches :enabled or :disabled; e.g., the
display and visibility properties have no effect
on the enabled/disabled state of an element.

Input elements can sometimes show placeholder text as a hint to the user
on what to type in. See, for example, the placeholder
attribute in [HTML5].
The :placeholder-shown pseudo-class
matches an input element that is showing such placeholder text.

While the :checked
pseudo-class is dynamic in nature, and can altered by user action, since
it can also be based on the presence of semantic attributes in the
document (such as the selected and checked attributes in [HTML5]), it applies to
all media.

An unchecked checkbox can be selected by using the negation
pseudo-class:

The :indeterminate pseudo-class applies
to UI elements whose value is in an indeterminate state. For example,
radio and checkbox elements can be toggled between checked and unchecked
states, but are sometimes in an indeterminate state, neither checked nor
unchecked. Similarly a progress meter can be in an indeterminate state
when the percent completion is unknown. For example, [HTML5] defines how checkboxes
can be made to match :indeterminate.

Like the :checked
pseudo-class, :indeterminate applies to all
media. Components of a radio-group initialized with no pre-selected
choice, for example, would be :indeterminate even in a
static display.

11.3. Input Value-checking

An element is :valid or :invalid when its contents or value is,
respectively, valid or invalid with respect to data validity semantics
defined by the document language (e.g. [XFORMS11] or [HTML5]). An element which lacks data
validity semantics is neither :valid nor :invalid.

Note that there is a difference between an element which has
no constraints, and thus would always be :valid, and one which has no data
validity semantics at all, and thus is neither :valid nor :invalid. In HTML, for example, an
<input type="text"> element may have no constraints, but a
<p> element has no validity semantics at all, and so it
never matches either of these pseudo-classes.

The :in-range and :out-of-range pseudo-classes apply only to
elements that have range limitations. An element is :in-range or :out-of-range when the value
that the element is bound to is in range or out of range with respect to
its range limits as defined by the document language. An element that
lacks data range limits or is not a form control is neither :in-range nor :out-of-range. E.g. a slider
element with a value of 11 presented as a slider control that only
represents the values from 1-10 is :out-of-range. Another example is a
menu element with a value of "E" that happens to be presented in a popup
menu that only has choices "A", "B" and "C".

A form element is :required or :optional if a value for it is, respectively,
required or optional before the form it belongs to can be validly
submitted. Elements that are not form elements are neither required nor
optional.

The :user-error pseudo-class represents
an input element with incorrect input, but only after the user
has significantly interacted with it. The :user-error pseudo-class must
match an :invalid, :out-of-range, or
empty-but-:required form
element between the time the user has attempted to submit the form and
before the user has interacted again with the form element. User-agents
may allow it to match such elements at other times, as would be
appropriate for highlighting an error to the user. For example, a UA may
choose to have :user-error
match an :invalid element once
the user has typed some text into it and changed the focus to another
element, and to stop matching only after the user has successfully
corrected the input.

For example, the input in the following document fragment would match
:invalid as soon as the page
is loaded (because it the initial value violates the max-constraint), but
it won't match :user-error
until the user significantly interacts with the element, or attempts to
submit the form it's part of.

12. Tree-Structural
pseudo-classes

Selectors introduces the concept of structural pseudo-classes to permit
selection based on extra information that lies in the document tree but
cannot be represented by other simple selectors or combinators.

Standalone text and other non-element nodes are not counted when
calculating the position of an element in the list of children of its
parent. When calculating the position of an element in the list of
children of its parent, the index numbering starts at 1.

The :empty pseudo-class represents an element
that has no children at all. In terms of the document tree, only element
nodes and content nodes (such as DOM [DOM-LEVEL-3-CORE] text
nodes, CDATA nodes, and entity references) whose data has a non-zero
length must be considered as affecting emptiness; comments, processing
instructions, and other nodes must not affect whether an element is
considered empty or not.

Several pseudo-classes in this section use a micro-syntax to indicate
indexes in a list of sibling elements. This syntax is referred to as the
An+B notation, and represents an integer step (A) and offset (B). It
indicates the An+Bth elements in a list,
for every positive integer or zero value of n, with the first
element in the list having index 1 (not 0).

For values of A and B greater than 0, this effectively divides the
list into groups of A elements (the last group
taking the remainder), and selecting the Bth
element of each group.

The An+B notation also accepts the
‘even’ and ‘odd’
keywords, which have the same meaning as ‘2n’
and ‘2n+1’, respectively.

Examples:

2n+0 /* represents all of the even elements in the list */
even /* same */
4n+1 /* represents the 1st, 5th, 9th, 13th, etc. elements in the list */

The values of A and B can be negative, but only the positive results
of An+B, for n ≥ 0, are used.

Example:

-n+6 /* represents the first 6 elements of the list */

If both A and B
are 0, the pseudo-class represents no element in the list.

12.4.1. Syntax

When A is 0, the An part may be
omitted (unless the B part is already
omitted). When An is not included and B is non-negative, the ‘+’ sign before B (when
allowed) may also be omitted. In this case the syntax simplifies to just
B.

If an "n" token is provided (the first clause in the grammar), the first
integer gives the value of the step. The
second integer, if provided, gives the value of the offset, defaulting to 0 if not provided.

If an "n" token is not provided (the second clause in the grammar), the
integer gives the offset, and the step defaults to 1.

If "odd" is provided, the step is 2 and
the offset is 1. If "even" is provided,
the step is 2 and the offset is 0.

12.5. Child-indexed
Pseudo-classes

The pseudo-classes defined in this section select elements based on
their index in their list of siblings.

The :nth-of-type(An+B) pseudo-class
notation represents an element that has An+B-1 siblings with the same expanded element
namebefore it in the document tree. See the An+B section for the definition of its
argument.

The :nth-last-of-type(An+B)
pseudo-class notation represents an element that has An+B-1 siblings with the same expanded element
nameafter it in the document tree. See the An+B section for the definition of its
argument.

Example:

To represent all h2 children of an XHTML body
except the first and last, one could use the following selector:

body > h2:nth-of-type(n+2):nth-last-of-type(n+2)

In this case, one could also use :not(), although the selector
ends up being just as long:

The :only-of-type pseudo-class
represents an element that has no siblings with the same expanded element
name. Same as ‘:first-of-type:last-of-type’
or ‘:nth-of-type(1):nth-last-of-type(1)’, but
with a lower specificity.

12.7. Selected
Child-indexed Pseudo-classes

The pseudo-classes in this section are also similar to Child Index Pseudo-classes, but they resolve based
on an element's index in the set of siblings that match a given selector.

A selector like ‘p.foo:nth-child(even)’ means "of all the even siblings,
select the <p> elements that have the class
foo", because simple selectors match independently, rather
than the sometimes-desired interpretation of "among the
<p> elements with class foo, select the even
ones". The ‘:nth-match()’ and ‘:nth-last-match()’ pseudo-classes allow one to
build a selector for the latter interpretation.

The :nth-match(An+B of
<selector>) pseudo-class notation represents an
element that has An+B-1 siblings that match
the given selector listbefore it in the document tree. See the An+B section for the definition of its first
argument.

The :nth-last-match(An+B of
<selector>) pseudo-class notation represents an
element that has An+B-1 siblings that match
the given selector listafter
it in the document tree. See the An+B
section for the definition of its argument.

13. Combinators

13.1. Descendant
combinator

At times, authors may want selectors to describe an element that is the
descendant of another element in the document tree (e.g., "an
EM element that is contained within an H1
element"). Descendant combinators express such a relationship. A descendant combinator is whitespace that separates two compound selectors. A selector of the form
"A B" represents an element B
that is an arbitrary descendant of some ancestor element A.

Examples:

For example, consider the following selector:

h1 em

It represents an em element being the descendant of an
h1 element. It is a correct and valid, but partial,
description of the following fragment:

represents a p element that is a grandchild or later
descendant of a div element. Note the whitespace on either
side of the "*" is not part of the universal selector; the whitespace is
a combinator indicating that the div must be the ancestor of
some element, and that that element must be an ancestor of the
p.

The following selector, which combines descendant combinators and attribute selectors, represents an
element that (1) has the href attribute set and (2) is
inside a p that is itself inside a div:

div p *[href]

13.2. Child combinator

A child combinator describes a childhood
relationship between two elements. A child combinator is made of the
"greater-than sign" (U+003E, >) character and
separates two compound selectors.

Examples:

The following selector represents a p element that is
child of body:

body > p

The following example combines descendant combinators and child
combinators.

div ol>li p

It represents a p element that is a descendant of an
li element; the li element must be the child of
an ol element; the ol element must be a
descendant of a div. Notice that the optional white space
around the ">" combinator has been left out.

For information on selecting the first child of an element, please see
the section on the :first-child pseudo-class above.

13.3.
Next-sibling combinator

The next-sibling combinator is
made of the "plus sign" (U+002B, +) character that
separates two compound selectors. The
elements represented by the two compound
selectors share the same parent in the document tree and the
element represented by the first compound
selector immediately precedes the element represented by the
second one. Non-element nodes (e.g. text between elements) are ignored
when considering the adjacency of elements.

Examples:

The following selector represents a p element immediately
following a math element:

math + p

The following selector is conceptually similar to the one in the
previous example, except that it adds an attribute selector — it adds a
constraint to the h1 element, that it must have
class="opener":

h1.opener + h2

13.4.
Following-sibling combinator

The following-sibling
combinator is made of the "tilde" (U+007E, ~)
character that separates two compound
selectors. The elements represented by the two compound selectors share the same parent in
the document tree and the element represented by the first compound
selector precedes (not necessarily immediately) the element represented by
the second one.

Example:

h1 ~ pre

represents a pre element following an h1. It
is a correct and valid, but partial, description of:

<h1>Definition of the function a</h1>
<p>Function a(x) has to be applied to all figures in the table.</p>
<pre>function a(x) = 12x/13.5</pre>

13.5. Reference
combinators

The reference combinator consists of
two slashes with an intervening CSS qualified
name, and separates two compound
selectors, e.g. A /attr/ B. The element represented
by the first compound selector explicitly
references the element represented by the second compound selector. Unless the host language
defines a different syntax for expressing this relationship, this
relationship is considered to exist if the value of the specified
attribute on the first element is an IDREF or an ID selector referencing the second element.
Attribute matching for reference combinators follow the same rules as for
attribute
selectors.

The following example highlights an <input> element
when its <label>
is focused or hovered-over:

14. Grid-Structural
Selectors

The double-association of a cell in a 2D grid (to its row and column)
cannot be represented by parentage in a hierarchical markup language. Only
one of those associations can be represented hierarchically: the other
must be explicitly or implicitly defined in the document language
semantics. In both HTML and DocBook, two of the most common hierarchical
markup languages, the markup is row-primary (that is, the row associations
are represented hierarchically); the columns must be implied. To be able
to represent such implied column-based relationships, the column combinator and the :nth-column() and :nth-last-column()
pseudo-classes are defined. In a column-primary format, these
pseudo-classes match against row associations instead.

14.1. Column combinator

The column combinator, which consists
of two pipes (‘||’) represents the relationship
of a column element to a cell element belonging to the column it
represents. Column membership is determined based on the semantics of the
document language only: whether and how the elements are presented is not
considered. If a cell element belongs to more than one column, it is
represented by a selector indicating membership in any of those columns.

The :nth-column(An+B) pseudo-class
notation represents a cell element belonging to a column that has An+B-1 columns before it, for
any positive integer or zero value of n. Column membership is
determined based on the semantics of the document language only: whether
and how the elements are presented is not considered. If a cell element
belongs to more than one column, it is represented by a selector
indicating any of those columns.

See :nth-child()
pseudo-class for the syntax of its argument. It also accepts the ‘even’ and ‘odd’ values as arguments.

The :nth-last-column(An+B)
pseudo-class notation represents a cell element belonging to a column that
has An+B-1 columns after
it, for any positive integer or zero value of n. Column
membership is determined based on the semantics of the document language
only: whether and how the elements are presented is not considered. If a
cell element belongs to more than one column, it is represented by a
selector indicating any of those columns.

See :nth-child()
pseudo-class for the syntax of its argument. It also accepts the ‘even’ and ‘odd’ values as arguments.

15. Calculating a selector's
specificity

count the number of class selectors, attributes selectors, and
pseudo-classes in the selector (= B)

count the number of type selectors and pseudo-elements in the selector
(= c)

ignore the universal selector

The specificity of a :matches() pseudo-class is
the specificity of the most specific complex selector that matched. (The
full selector's specificity is equivalent to expanding out all the
combinations in full, without :matches().) The specificity of a
:not() pseudo-class is the specificity of the most
specifc complex selector in its selector list. In either case, the
pseudo-class itself does not contribute any additional specificity. For
example, :matches(em, strong) has a specificity of (0,0,1),
like a tag selector.

Specificities are compared by comparing the three components in order:
the specificity with a larger A value is more
specific; if the two A values are tied, then
the specificity with a larger B value is more
specific; if the two B values are also tied,
then the specificity with a larger c value is more specific; if
all the values are tied, the two specifities are equal.

Due to storage limitations, implementations may have limitations on the
size of A, B, or
c. If so, values higher than the limit must be clamped to that
limit, and not overflow.

16.
Grammar

The grammar below defines the syntax of Selectors. It is applied to a
stream of tokens, as returned by the tokenizer defined in [CSS3SYN]. It is
globally LL(1) and can be locally LL(2) (but note that most UAs should not
use it directly, since it doesn't express the parsing conventions). The
format of the productions is optimized for human consumption and some
shorthand notations beyond Yacc (see [YACC]) are used:

*: 0 or more

+: 1 or more

?: 0 or 1

|: separates alternatives

[ ]: grouping

The productions in uppercase are defined by CSS Syntax [CSS3SYN], and
correspond to the tokens of the same name. Literal strings correspond to
delim tokens with the given value. The production "S" represents a
whitespace token. The wqname_prefix production comes from the Namespaces
spec [CSS3NAMESPACE] The

18. Conformance

18.1. Document Conventions

Conformance requirements are expressed with a combination of descriptive
assertions and RFC 2119 terminology. The key words “MUST”, “MUST
NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”,
“SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the
normative parts of this document are to be interpreted as described in RFC
2119. However, for readability, these words do not appear in all uppercase
letters in this specification.

All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for
example” or are set apart from the normative text with
class="example", like this:

This is an example of an informative example.

Informative notes begin with the word “Note” and are set apart from
the normative text with class="note", like this:

Note, this is an informative note.

18.2. Conformance
Classes

Conformance to Selectors Level 4 is defined for three conformance
classes:

A selector instance is conformant to Selectors Level 4 if it is valid
according to the selector syntax rules defined in this specification.

An interpreter is conformant to Selectors Level 4 if it parses
interprets selectors according to the semantics defined in Selectors Level
4 (including following the error-handling rules). However, the inability
of a user agent to implement part of this specification due to the
limitations of a particular device (e.g., non interactive user agents will
probably not implement dynamic pseudo-classes because they make no sense
without interactivity) does not imply non-conformance.

An authoring tool is conformant to Selectors Level 4 if it writes
syntactically correct selectors.

Any specification reusing Selectors must define which subset of Selectors
it accepts or excludes, and describe any constraints it adds to the current
specification.

Specifications reusing Selectors must define how to handle invalid
selectors. (In the case of CSS, the entire rule in which the selector is
used is effectively dropped.)

18.3. Partial Implementations

So that authors can exploit the forward-compatible parsing rules to
trigger fallback behavior, UAs must treat as invalid any selectors for which they have no usable
level of support.

18.4. Experimental
Implementations

To avoid clashes with future Selectors features, the Selectors
specification reserves a prefixed
syntax for proprietary extensions to Selectors. The CSS Working Group
recommends that experimental implementations of features in Selectors
Working Drafts also use vendor-prefixed pseudo-element or pseudo-class
names. This avoids any incompatibilities with future changes in the draft.
Once a specification reaches the Candidate Recommendation stage,
implementors should implement the non-prefixed syntax for any feature they
consider to be correctly implemented according to spec.

19. Acknowledgements

The CSS working group would like to thank everyone who contributed to
the previous Selectors
specifications over the years, as those specifications formed the basis
for this one.

In particular, the working group would like to extend special thanks to
the following for their specific contributions to Selectors Level 4: L.
David Baron, Andrew Fedoniouk, Ian Hickson, Grey Hodge, Lachlan Hunt,
Jason Cranford Teague